Pension Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Components of Net Periodic Benefit Costs for Both Pension Plans
Service cost	$ 2,113	$ 2,390	$ 4,100	$ 3,974
Interest cost	830	700	1,454	1,248
Expected return on plan assets that reduces periodic pension costs	(573)	(597)	(1,118)	(1,091)
Amortization of prior service costs	5	5	9	9
Amortization of net actuarial loss	419	571	791	743
Net periodic benefit cost	$ 2,794	$ 3,069	$ 5,236	$ 4,883